1. Field of the Invention
The present invention relates to an internal combustion engine including an exhaust gas recirculation (EGR) cooler that exchanges heat between exhaust gas that is recirculated to a combustion chamber through an EGR mechanism and engine coolant for cooling the recirculated exhaust gas.
2. Description of the Related Art
Some internal combustion engines including an EGR mechanism that recirculates a portion of exhaust gas to a combustion chamber to reduce nitrogen oxides (NOx) contained in the exhaust gas are known to include an EGR cooler that cools the exhaust gas to be recirculated via the EGR mechanism through heat exchange with engine coolant (for example, see Japanese Patent Application Publication No. 2000-45884 (JP-A-2000-45884)).
As shown in FIG. 5, such an EGR cooler is provided in an intermediate part of an EGR passage 2, which branches from an exhaust passage of the internal combustion engine and is connected to an intake passage. A liquid chamber 3 is provided as a coolant passage inside the EGR cooler. To the liquid chamber 3 is connected a flow-in passage 4, which is connected to a coolant passage of the internal combustion engine to allow the engine coolant to flow into the liquid chamber 3 therethrough as indicated by an arrow in FIG. 5. The engine coolant having flowed into the liquid chamber 3 is returned to the coolant passage of the internal combustion engine through a flow-out passage 5 as indicated by an arrow in FIG. 5. Also as shown in FIG. 5, a plurality of exhaust cooling passages 6 are provided in the EGR cooler that pass in the liquid chamber 3 to connect an EGR passage 2A on the exhaust passage side and an EGR passage 2B on the intake passage side.
This allows the exhaust gas, having flowed into the EGR cooler through the EGR passage 2A on the exhaust passage side, to flow into the EGR passage 2B on the intake passage side through the exhaust cooling passages 6, which extend in the liquid chamber 3 in which the engine coolant flows. Consequently, heat is exchanged between the engine coolant flowing in the liquid chamber 3 and the exhaust gas flowing through the exhaust cooling passages 6.
As described above, inside the EGR cooler in which a coolant passage through which engine coolant flows and a passage for exhaust gas are provided adjacently, heat is exchanged between the engine coolant and the exhaust gas via a partition wall between the respective adjacent passages. Consequently, the exhaust gas is cooled before being recirculated to improve the charge efficiency of the exhaust gas into the combustion chamber, and the temperature of combustion in the combustion chamber is reduced to more efficiently suppress the generation of NOx.
In recent years, however, it has been desired to recirculate the exhaust gas to the combustion chamber even during cold engine operation, in order to further improve the exhaust properties, or to comply with stricter emission regulations, or for other reasons. However, because the temperature of the engine coolant is extremely low during cold engine operation, the difference in temperature between the exhaust gas and the engine coolant flowing through the EGR cooler is significantly large. As a result, the recirculated exhaust gas is abruptly cooled in the EGR cooler during cold engine operation, which may cause water contained in the exhaust gas to condense in the EGR cooler to result in the generation of a large amount of condensed water in the EGR cooler.
Once water has condensed in this way, the EGR cooler may become corroded and the condensed water may enter the intake passage side and be recirculated along with the exhaust gas. This may result in various inconveniences, freezing of the condensed water when the engine is stopped to thereby block the induction port of the intake passage for the exhaust gas, or like that intake valves become stuck.